This invention is related to the problem of providing a high wear, pick-up, and thermal shock resistant coating for hearth rolls for annealing steel, stainless steel and silicon steel sheet in a continuous annealing furnace. The hearth rolls carry the steel sheet through the furnace. The temperature in the furnace may vary from about 1500.degree. to over 2000.degree. F. depending upon the type of steel, the travel speed of the sheet steel as it passes through the furnace and the duration of time in the furnace.
A major problem encountered in the annealing operation is the transfer or pick-up of material from the steel sheet to the hearth rolls. If pick-up occurs, it will accumulate on the hearth rolls and damage the steel sheet being processed. To avoid this problem frequent roll changes are required with concomitant costs for replacement and lost production. This problem has become more severe in recent years since higher speeds and temperatures are being used to increase productivity.
To suppress the transfer of material to the hearth roll and to increase wear resistance, it is desirable to coat the hearth roll with a coating composition which is substantially chemically inert at elevated temperatures. An undercoating of metal or a ceramic-metal alloy is used to prevent spalling when there is an excessive mismatch in thermal expansion between the coating and the substrate. Spalling may also be prevented using a graded or multilayer under-coating in which the composition of the undercoat is gradually varied from 100% alloy to 100% ceramic.
Japanese Patent No. 563-26183 discloses a hearth roll coated with zirconia partially stabilized by yttria. This coating has good pick-up resistance and thermal shock resistance, but is difficult to produce with high density and good wear resistance. Japanese Patent No. 563-50428 discloses zirconia containing silica as a coating for hearth rolls. However, this costing exhibits excessive pick-up and microspalling.
U.S. patent application Ser. No. 596,896, filed on Oct. 11, 1990, discloses a feed powder composition for use as a coating for hearth rolls which comprises particles of zirconium silicate (zircon) and particles of stabilized or partially stabilized zirconia. During the thermal deposition of the feed powder, the zircon is deposited as zircon and/or its decomposition products SiO.sub.2 and ZrO.sub.2.
It is an object of this present invention to provide a refractory oxide coating for use on hearth rolls that will have high thermal shock resistance, superior wear resistance, and excellent resistance to pick-up.
It is another object of the present invention to provide a refractory oxide coating for use on hearth rolls that will exhibit good crystallographic characteristics when subject to thermal cycling environments.
It is another object of the present invention to provide a process for producing a refractory oxide coating ideally suitable for use on hearth rolls for annealing steel.